This invention relates to a laser diode element and, particularly, to a Distributed Feed Back (DFB) laser diode element.
A conventional laser diode element of the type described generally comprises a semiconductor block having a front facet, a rear facet opposite the front facet, a laser cavity formed between the front and the rear facets and having a predetermined length L, a plurality of coating layers coated on the front and the rear facets to provide a predetermined reflectively, respectively, an active layer and a grating formed in the direction of the laser cavity and coupled to each other at a predetermined coupling constant K.
Many types of the DFB laser diode elements have been developed to have good single longitudinal mode characteristics.
For example, some DFB laser diode elements were proposed in laid-open Japanese patent application No. H1-155677, H2-90688, and H2-20087. In these examples, the predetermined reflectively as well as a product of the predetermined coupling constant K and the predetermined length L in each DFB laser diode element are optimized from each point of view.
On the other hand, another DFB laser diode element has been proposed in laid-open Japanese patent application No. Sho 62-219684. As illustrated in the drawing of the laid-open paper, the DFB laser diode element includes a partial grating having a regular corrugation kept in contact with the front facet.
In the interim, a DFB laser diode element has recently been used as a light source for analog modulation in a subcarrier multiplexing optical transmission method, or the like.
When used as such a light source for the analog modulation, the DFB laser diode element is required to have a better intermodulation distortion characteristic. For example, in the field of a mobile communication system, the DFB laser diode element is desired to have a characteristic in which 3rd intermodulation distortion (IMD.sub.3) is sufficiently small.
However, none of the conventional laser diode elements mentioned above has ever been fabricated for the purpose of analog modulation. Accordingly, no consideration has been given to intermodulation distortion characteristics in the conventional laser diode elements. As a result, even when the conventional laser diode elements are used as light sources for analog modulations, the yield in getting laser diode elements with sufficient intermodulation distortion characteristics is inevitably not very good.